Simplify the "Adding documentation" article for godot-cpp, and de-duplicate format information.

[Ivorforce]

The current documentation for documentation in godot-cpp covers everything that's necessary, but there are several things I'd like to improve:

• The information about version compatibility is too extensive, and distracts from important information. People will either use 4.3 or newer, and read the article, or use 4.2 or lower, and skip it via the disclaimer.
• The disclaimer about assumptions of the article (e.g. with the file structure) is not needed. The actual assumptions we need are just that there is a demo folder. Listing more assumptions than needed distracts from the contents, and might unnecessarily scare away potential readers.
• The Documentation styling section is duplicate to the "Class Reference Primer". The format used by Godot's class reference is the exact same as it is for GDExtension. We do not need this information twice.
• I adjusted wording to the new structure, and hopefully made it easier to read.
• I've also added a link to the documentation guidelines. While it's written for potential contributors, it's also a good reference for writing docs for GDExtensions.

[Ivorforce]

For quick reference, here's the (current) new version:

[paddy-exe]

I think in this case we should just say "identify your project's demonstration or testing project folder" since otherwise I see some people being confused that they need a demo folder

[Ivorforce]

How about "identify your project's test project folder"?

[paddy-exe]

This should not be deleted as there is an overlap with the general RichText documentation but not every tag works in the GDExtension documentation. I tested every tag and wrote down each one that works.

[Ivorforce]

I added a link to Class reference primer, which is what these files are. It is unrelated to the RichtText format (although there is some overlap).

[paddy-exe]

I see. There's no img or colorsupport in the class reference primer though as I see but are supported by the GDExtension docs system though.

[Ivorforce]

Perhaps the documentations are just missing this piece of information?
Since I think it's using the same system, I'd be surprised if there were features in GDExtension docs that aren't supported by Godot's class reference.

[paddy-exe]

What is meant here by personal insight? That seems unclear

[Ivorforce]

The idea was to communicate that the tutorial is not "complete", and whoever wants to publish their documentation will need to put in additional work beyond what the tutorial covers. Phrasing it like this was suggested back when this was added.

[Calinou]

Suggested change

	The if-statement avoids adding the documentation to release builds of your GDExtension, where it is not needed.
	SCons then loads all the XML files inside the ``doc_classes`` directory, and appends the resulting targets
	to the ``sources`` array, to be included in your GDExtension build.
	The ``if`` statement avoids adding the documentation to release builds of your GDExtension, where it is not needed.
	SCons then loads all the XML files inside the ``doc_classes`` directory, and appends the resulting targets
	to the ``sources`` array, to be included in your GDExtension build.

[Calinou]

Suggested change

	You can use the repository as a basis to build your own documentation system.
	The following guide describes the basic steps, but they are not exhaustive:
	You will need a bit of personal insight to make it work.
	You can use the repository as a basis to build your own documentation system.
	The following guide describes the basic steps, but they are not exhaustive;
	you will need a bit of personal insight to make it work.